Cellulose acetate tow producers market uniform pressure drop (PD) to cigarette filter producers. Tow, however, is sold by weight. The relationship between PD and weight is referred to as yield (PD/weight). Yield is often illustrated by a line on a graph where the x-axis is the weight and the y-axis is the PD. The lowermost end of the yield line is defined as the point at which the rod develops recessed ends and the uppermost end of the yield line is defined as the point at which the rod splits or machine roll wraps occur because of too much tow. Browne, C. L., The Design of Cigarettes, Hoechst Celanese Corporation, 1990, page 66.
The cigarette filter is a very complex device and many factors effect its production and performance. As with all complex devices, these factors are often interrelated, so that changes in one factor have effects on other factors. Several factors, specifically addressed herein, include firmness, pressure drop, PD variability, fly, and openability. These qualities are considered by a filter producer when comparing tow suppliers. Firmness, a rod quality, refers to the deformation of a filter rod under a specified load for a specified contact time. The load cell weight and contact time is dependent on the instrument used. Firmness is generally expressed as the percentage of diameter retained (i.e., a higher percentage is more desirable). PD variability, a rod quality, refers to the PD uniformity of a large number of rods and is quantified by a Cv (coefficient of variation). Filter producers want the lowest possible Cv to achieve minimum variability in the delivery of cigarette smoke components. Fly, also called “lint”, a tow quality, is not often quantified, but is readily apparent to the filter producer while removing tow from the bale or on the rod-making machine, and can be a significant source of defective filter rods (lumps of fiber, wormholes) as well as a cause for more frequent cleaning the opening and rodmaking machinery. Openability, a tow quality, refers to the ease of opening in the rodmaking equipment to completely deregister, or “bloom”, the tow, and is seldom quantified, but is readily apparent to the filter producer.
Obviously, the filter producer wants a tow product that provides a rod that possesses the desired firmness and low PD variability, opens easily, and has no fly. With the current state-of-the-art, such a product is not available. Moreover, the route to producing this product is not clear due to the complexities associated with the production of cigarette filters and cigarette filter tow.
One skilled in the art knows that firmness, pressure drop, PD variability, fly, and openability can be influenced by tow crimp. Crimp is a waviness imparted to synthetic fibers during manufacture and crimp level may be measured as uncrimping energy (UCE). One skilled in the art recognizes that influencing crimp to improve one quality often causes another quality to suffer. For example, increasing UCE increases fly (bad), and decreases PD variability (good), and inhibits openability (bad), other process conditions generally remaining unchanged.
Products with extremely high crimp have been produced, but are not problem free. For example, Rhodia Acetow® produces a product under the tradename Rhodia SK®. Rhodia SK® is a high yield tow (meaning high PD for low weight) and achieves that result with high crimp. But, Rhodia SK also has greater than normal fly and is difficult to open at conditions typical for conventional tow. This follows the conventional wisdom. The difficulty associated with opening is seen by the requirement to change from conventional rodmaking settings, i.e., increased work must be applied to the tow to completely deregister, or “bloom”, the tow which may be accomplished by changes in the threaded roll design, the threaded roll pressure, and/or the ratio of roll speed on the rodmaker. This increased work results in additional fly due to fiber breakage.
Accordingly, the problem is how to produce a tow product that opens easily and provides a filter rod with the desired firmness, low PD variability, and low fly. Based upon the prior art, such a product cannot be obtained solely by a high crimp tow.
U.S. Pat. No. 3,353,239 discloses a stuffer box crimper where the nip rollers have circumferential grooves.
Japanese Patent No. 2964191 (based on Japanese Application No. 1991-358234 filed Dec. 27, 1991) is directed to a stuffer box crimper for cigarette tow production. This patent teaches that lubricating the edges of the tow prior to crimping with a lubricant (i.e., water) at a feed rate of 25–50 cc/min will reduce fly.
U.S. Pat. No. 3,305,897 discloses steam crimping of polyester tow in a stuffer box crimper. Steam at 20–40 psig is introduced into the stuffer chamber. U.S. Pat. Nos. 5,225,277 and 5,618,620 disclose heat-treating the tow with steam upstream of the crimper or while the tow is in the crimper. Japanese Application No. 54-127861 discloses heat treatment of tow upstream of the crimper. U.S. Pat. No. 5,591,388 discloses a process for producing crimped lyocell (solvent-spun cellulose) using slightly superheated (dry) steam injected onto the fibers as they are crimped in the stuffer box of a crimper. The superheated steam is at a pressure of 5 psi to 70 psi or greater.
WIPO Publication No. WO 02-087366 illustrates that increasing crimp levels also increases the fly (fluff) of the tow. Note Examples.